Please refer to FIG. 1A. FIG. 1A is a diagram illustrating a burst cutting area (which is called as BCA in the following descriptions) on an optical disc. The BCA is a circular and is near to a center hole of the optical disc, wherein the pattern of the BCA is similar to a bar code and alternates bright with dark. Generally speaking, The BCA is an area used for recording copyright-related information on the optical disc, which records essential messages capable of decoding contents of the optical disc. As a result, the BCA is widely applied to an optical disc having copyright protection. For a latest blue-ray disc, the BCA is more than an essential protection mechanism. Hence, how to quickly and accurately decode the data in the BCA has become an important problem that needs be solved by disc drive manufactures.
Please refer to FIG. 1B. FIG. 1B is a diagram illustrating the structure of a BCA defined in the blue-ray disc specification. Each row has five bytes, wherein a first byte of each row is a sync byte SBBCA and the sync byte SBBCA of each row has the same sync pattern. Based on the blue-ray disc specification, the first sync byte (SBBCA,-1) is the beginning of the whole BCA, so that the whole BCA can be decoded only when the first sync byte (SBBCA,-1) is first detected. In addition, based on the blue-ray disc specification, a BCA signal with 5 T, 3 T, 2 T, and 4 T is generated when the laser beam emitted from the optical pickup head is projected on the sync pattern. In other words, when a pattern with 5 T, 3 T, 2 T, and 4 T is appeared in the BCA signal, it represents that the sync pattern is detected in the BCA signal.
Since the BCA signal is affected by the rotating speed of the spindle motor, the conventional methods usually find out an accurate sampling clock first and then utilize the sampling clock to perform the follow-up sync pattern detection in the BCA signal and the decoding of the BCA signal in order to accurately sample the BCA signal and the sync pattern of the BCA signal.
A sampling clock generator for BCA data decoding has been disclosed in the U.S. Pat. No. 7,006,415. As shown in FIG. 2, the sampling clock generator 41 receives a spindle-motor-speed indicating signal FG and a BCA reproducing signal, and then generates a sampling clock in response to the spindle-motor-speed indicating signal FG and the BCA reproducing signal. After that, the sampling clock is inputted to a BCA channel bit generator 42 and a BCA decoder 43. As a result, the BCA channel bit generator 42 is capable of finding out the sync pattern of the BCA signal according to the sampling clock and accurately generating a BCA channel bit to the BCA decoder 43. The BCA decoder 43 is capable of generating a BCA data bit according to the sampling clock and the BCA channel bit.
Obviously, the spindle motor must be fixed at a designated rotating speed in the above-mentioned conventional method, and thus the sampling clock can be calculated according to a relationship between the BCA reproducing signal and the rotating speed. However, the frequency variations of the sampling clock are very small and will be easily affected by the optical disc or the spindle motor.
A method and apparatus for generating a sampling clock for a burst cutting area of an optical disc has been disclosed in the US Publication No. 20060181998. As shown in FIG. 3, the sampling clock generating device 400 includes a BCA signal reproduction device 410, a detecting device 420, a computing unit 430, a clock generator 440, and a control unit 450.
After a radio frequency signal passes through the BCA signal reproduction device 410, a BCA reproducing signal BRS can be generated to the detecting device 420. A counter 422 of the detecting device 420 then continuously counts the BCA reproducing signal BRS according to a reference clock REFCLK to obtain a counting value, and the counting value is stored in a register 424 of the detecting device 420. In addition, when the counting value of the counter 422 is greater than the counting value stored in the register 424, the counting value stored in the register 424 will be updated. That is, the counting value stored in the register 424 is the largest value. Moreover, based on the blue-ray disc specification, the maximum channel bit length of the BCA reproducing signal BRS is 5 T.
For example, assume that the counting value stored in the register 424 is 100. The counting value is divided by 5 by using the computing unit 430 (i.e., 100/5=20), so as to output a value equaling 20 to the clock generator 440. The clock generator 440 obtains a period of the channel bit length with 1 T equaling twenty periods of the reference clock REFCLK, and generates the sampling clock. Only after the sampling clock is obtained, the sync pattern detection can be performed according to the sampling clock.